Coupling network



NGV 3, 1942- s. DARLINGTON COUPLING NETWORK Filed July 25, 1941 PatentedNov. 3, 1/942 COUPLING NETWORK Sidney Darlington, New York, N; Y.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application July 25, 1941, Serial No. 403,959l18 Claims. '(Cl. 17d-44) This invention relates to Wave transmissionnetworks and more particularly to a network for coupling two unequalload impedances.

An object of the invention is to connect two loads of unequal impedance.

Other objects are to widen the band transmitted by a network forcoupling unequal` load impedances, reduce the transmission loss in thehigher frequency range and minimize the phase shift introduced by such anetwork and reduce the reflection eiects at the point of juncturebetween the network and the load.

In a. wave transmission system, it is often required to connect togethertwo unequal load impedances while introducing the minimum of highfrequency transmission loss, phase shift and impedance mismatch.Heretofore, it has been diflicult to accomplish these objectives,especially if the system is to transmit a wide band of frequencies suri,for example, as is employed in television.

In accordance with the invention, there is provided a coupling networkof comparatively simple structure which will meet all of therequirements set forth. The network comprises two inductivelyA coupledinductors, connected in the series-opposing relationship in anelectrical path between the two unequal load impedances, and animpedance branch including a capacitor and a resistor in parallel,connected from a point between the inductors to a point in a secondelectrical path between 4the load impedances. In the lower frequencyrange, where the impedance of the inductors is low and the impedance ofthe capacitor is high, the network functions as a resistance coupling.In the higher frequency range, where the impedance of the capacitor islow compared to that of the resistor, the network acts as a transformercoupling. The elements may be so proportioned that one of the loads isterminated in an impedance which is substantially matching over theentire frequency range.

As compared to a simple resistance coupling the network provides atransmission gain which increases with frequency, at first more rapidlyand later less rapidly. The frequency range covered depends upon thechoice of values for the inductors and the capacitor.

The network may be constructed in either the balanced orv the unbalancedform. Blocking capacitors may be added between the coupled inductors toprevent the passage of direct current. In order to annui the effects ofthe leakage inductance and the stray capacitances, a shunt lter section.To annui the effect of a negative reactive component which may beassociated with one of the load impedances a series inductor may beadded to form another low-pass filter section.

The knature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawing in which like reference characters represent like or similarpartsl and in which:

Fig. 1 is a schematic circuit of a coupling network in accordance withthe invention;

Fig. 2 shows a family of curves representing the transmission gainintroduced by the network, as compared to a resistance coupling; and

Fig. 3`is a schematic circuit of the coupling network in balanced form.

Taking up the gures in more detail, Fig. 1

shows one embodiment of the coupling network y of the invention in itsunbalanced form. The impedances Ri'and Rz connected respectively to thetwo pairs of terminals i, 2 and 3, 4 represent the unequal loads whichare to be connected capacitor may be added to provide a low-passtogether. The load R1 may, for example, be a transmission line or otherutilization circuit and R2 may be an amplifier or other terminal appa.-ratus, the impedance of which is higher than that of the load R1. In anelectrical path between the terminals l and 3 are two closely coupledinductors L1 and L2 connected in the series-opposing relationship.Connected from a pointv 5 between the inductors L1' and L2 to a point 6in an electrical path between the terminals 2 and 4 is an impedancebranch comprising a resistor R and a capacitor C in parallel.

In order to avoid reilection eiiects at the junction of the line orother load R1 and the network, it is desirable that the load R1 beterminated in a matching impedance. At low fre,- quencies the impedanceof the inductors L1 and L2 Will be small and the impedance of thecapacitor C will be large, leaving the shunt resistor R asr the onlyeffective impedance. The resistor R is, therefore, given ,such a valuethat, in parallel with the load Rz, the combination will have animpedance equal to R1. Assuming that the impedances R1 and R2 arenon-reactive, the required value of the resistance of R is found fromthe equation:

RIRZ

R-R2R1 When R has this'value, the impedance of the network at theterminals l and 2 will be equal to R1 and, therefore, the load R1 willbe terminated in its own impedance in the low frequency range.

At high frequencies the impedance of the capacitor C is small and theresistor R is effectively shunted out, leaving only"th\ '\V4 transformeraction of the inductors L1 and I a. ,For-proper impedance matching the'inductances of L1 and Lz should, therefore, have the same ratio as thatof the loads R1 and R1. Assuming perfect coupling between the inductors,

impedance of the network at thetgrminals I and 2 will be equal to R1over ,diiglilfrequency range.

If the capacitance of h'icapacitprac is propeny chosen, the network willalso have' an impedance at the terminals I and 2 which is approximately`equal to R1 over the intermediate frequency range. The required valueof C is given-by the equa tion s, l i 3 4 RlRg rf the inductors L1 andLe have ,thif'sfrand the i The only factor yet to be `determined is theinductance oi' the inductor L1, the choice of which depends upon thefrequency range to be covered. 'Ihe curves of Fig. 2 show on alogarithmic frequency scale, the transmission gain in decibels resultingfrom the insertion of the coupling network of Fig. l, as compared withthe insertion of only the shunt resistor R. Curve 8 represents this gainif L1 has a certain value. The curve rises with frequency, at first moreand then less rapidly. If L1 is doubled, for example, the gaincharacteristic will be as shown by curve 9, and

if L1 is halved curve III will result.,A It is seen, therefore, that byproperly choosing the value of L1 any one of an inilnlte family of gaincharacteristics may be obtained In practice' L1 `may often be so chosenthat the resulting gain characteristic serves to equalize to a largeextent for attenuation distortion in other parts of the system such, forexample, as an associated transmission line. It should be pointed out,further, that the coupling network oi' Fig. 1 introduces the minimumphase 1 shift that it is possible to associate with a given insertionloss characteristic.

The schematic circuit of Fig. 3 shows a balanced form of the couplingnetwork of the invention.

thermionic tubes connected in series and working plate impedances or thegrid irnpedances of,.two

push-pull. These tube lrnpedances are ordinarily to the'tubes includedin the load Rz.'

resistance of value R tapped at its mid-point M and grounded as shown atI5. sistors Ra and R4, of comparatively highA resistance,` vareconnected between the inner terminaisV I6 and I1 of the two halves ofLe;4 Tl`se resistors have a common terminal Ii'which may be used, forexample, in supplying plate current Terminals I2 and I6 are connectedthrough a capacitor C1 and terminals I3and I'l are connected through acapacitor Cz. These capacitors have large capacitances and theirfunction4 `is to provide a low impedance path at high frequencies but toprevent the flow of direct current through the network.

The shunt capacitor C: is proportioned with 'respect to the leakageinductanceiofnthe coupled inductors and the associatedstrayfpapacitances to form a low-pass filter section with-"cut-otf abovethehighest frequency to be transmitted by the network. The two equalseries inductors L: and 'L4 are proportioned with respect to thenegative reactive component associated with the load Rz tol provide 4asecond low-pass filter section also having a cut-oli.' above the highestfrequency of interest. These filter sections are so designed that theirimage impedances approximate the impedance Rz over the transmittedrange.

These added elements Ca, In and L4 serve to make the impedance of thenetwork at terminals work of Fig. 3 may be designed to transmitsatisfactorily the broad band of frequencies used in television,extending, for example, from 30 cycles or lower to 5 megacycles orhigher. When used in television circuits the minimum phase shiftproperty of the network, already mentioned, is of 'considerableimportance as it materially simplifies the problem of phase correction.

What is claimed is:

1. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, twoinductively coupled inductors connected in the series-opposingrelationship in one of said paths and an impedance branch including acapacitor and a resistor in parallel connected from a point in said onepath between said inductors to a point in said other path, theinductance ratio o'f said inductors being approximately equal to theratio of said load impedances.

2. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, twoinductivelylcupled conductors connected in the series-opposingrelationship in one of said paths and an impedance branch including acapacitor and a resistorin parallel connected from a point iiisaid-"onepath between said inductors to a point in said. other path, saidresistor having a resistance approximately equal to the product of saidlead impedances divided by the difference shunted by a resistance toreduce and sta'giilize ""f Said:impedanes the impedance R2. As shown inFig. 3 each of the windings L1 Lz of the transformer is dividedf'intotwo 4equal parts. Between the inner terminals I2 and'l3 uf. the twohalves off` L1 is connected the impedance branch comprising thecapacitor C shurrfed by a `z 3. A', networkzfor coupling two unequalload impedances comprising tufo electricalj' paths adapted tointerconnect saidlimpedarrces, two inductively coupled conductorsconnected in the series-opposing relationship-in one of, said paths andan; impedance branch'mcluding a capacitor Two equal re4 and a resistorin parallel connected from a point in said one path between saidinductors to a point in said other path, the capacitance C of saidcapacitor having approximately the value and an impedance branchincluding a capacitor and a resistor in parallel connected from a pointin said one path between said inductors to a point in said other path,the capacitance of said capacitor being so proportioned with respect tothe inductance of one of said inductors and the values of said loadimpedances that the network has at one end an impedance which, over nwide 'range offrequencies, is approximately equal to the load impedanceat that end.

5. A network for coupling two unequal load impedances comprising twoelectrical' paths adapted to interconnect said impedances, twoinductively coupled conductors connected in the series-opposingrelationship in one of said paths andan impedance branch including acapacitor and a resistor in parallel connected from a point in said onepath between said inductors to a point in said other path, said resistorhaving a resistance approximately equal to the product of said loadimpedances divided by the difference of said impedances and theinductance ratio of said inductors being approximately equal to theratio of said impedances.

6. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, twoinductlvely coupled conductors connected in the series-opposingrelationship in one of said paths and an impedance branch including acapacitor and a resistor in parallel connected from a point in said onepath between said inductors to a point in said other path, said resistorhaving a resistance approximately equal to the product of said loadimpedances divided by the difference of said impedances and thecapacitance of said capacitor being so proportioned with respect to theinductance of one of said inductors and the values of said loadimpedances that the network has at one end an impedance which isapproximately equal to the load impedance at that end.

7. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, twoinductively coupled conductors connected in the series-opposingrelationship in one of said paths and an impedance branch including acapacitor and a resistor in parallel connected from a point in said onepath between said inductors to a 4point in Said other path. theinductance ratio of said inductors being approximately equal to theratio of said load impedances and the capacitance of said capacitorbeing .so proportioned with respect to the inductance of one of saidinductors and the values of said load impedances that the network has atone end an impedance which is approximately equal to the load impedanceat that end.

8. A network for coupling two unequal load transmitted by the network.

impedances comprising two electrical paths adapted t'o interconnect saidimpedances, two inductively coupledl-.conductors connected in theseries-opposing'elationship in one of said paths and an impedance branchincluding a capacitor and a resistor inparallel connected from a pointin said-fr path between said inductors to a point in said otherl path,said resistor having a resistance approximately equal to the product ofsaid load impedances divided by the difference of said impedances, theinductance ratio of said inductors being approximately equal to theratio oi said impedances and the capacitance of said capacitor being soproportioned with respect to the inductance of one of said inductors andthe values of said load impedances that the network has at one end animpedance approximately equal to the load impedance at that end.

9. A network in accordance with claim 1 and an added capacitor connectedin shunt at one end of the network, the capacitance of said addedcapacitor being proportioned with respect to the leakage inductance 'ofsaid inductors and the stray capacitances to provide a low-pass `filterhaving a cut-olf above the highest frequency to be transmitted by thenetwork.-

10. A network in accordance' with claim 1 in which inductance is addedin serieswith one o1' said induotors, the value of said inductance beingproportioned with respect to a negative reactive component associatedwith one of said.

11. A network inA accordance with claim 1 which includes an addedcapacitor connected in shunt at one end of said network and addedinductance in series with one of said inductors, the capacitance of saidAadded capacitor being proportioned with respect to the leakageinductance oi said coupled inductors and the stray capacitances, and thevalue of said added inductance being proportioned with respect to anegative reactive component associated with one of said load impedances,to provide a low-.pass lter having a. cut-oi! above the highestfrequency to be transmitted by the network.

12. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, a pair ofinductively coupled inductors connected in the series-opposingrelationship in one of said paths, a second similar pair of inductorssimilarly connected in the other of said paths and an' impedance branchincluding a capacitor and aresistor in parallel connected from a pointin said one path between said first pair of inductors to a point in saidother path between said second pair of inductors, the inductance ratioof the inductors forming each of said pairs being approximately equal tothe ratio of said load imped- BfnCeS.

13. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, a pair ofinductively coupled inductors connected in the series-opposingrelationship in one of said paths, a second similar pair of inductorssimilarly connected in the other of said paths andan impedance branchincluding a capacitor and a resistor in parallel connected from a pointin said one path between said first pair of inductors to a point in saidother path between said second pair of inductors, said resistor having aresistance approximately equal to the product of said load iin--pedances divided by the diierence of said impedances.

14. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, a pair ancebranch including a capacitor and a resistor;y

in parallel connected from a point -in---s'aid one path between said rstpair of inductors to a. point in said other path between said secondpair of inductors, the capacitance of said capacitor being soproportioned with respect to the inductance of one of said inductors andthe values of said load impedances that the network has at one end animpedance which, over a wide range of frequencies, is approximatelyequal to the load impedance at that end.

15. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances, a pair ofinductively coupled inductors connected in the series-opposingrelationship in one of saidpaths, a second similar pair of inductorssimilarly connected in the other of said paths and an impedance branchincluding a capacitor and a resistor in' parallel connected from a pointin said one path between said first pair of inductors to a 4point insaid other path between said second pair of inductors, the two inductorsin each of said pairs being connected through an added capacitor.

16. A network for coupling two unequal load impedances comprising twoelectrical paths adapted to interconnect said impedances. a pair ofinductively coupled inductors connected in the series-opposingrelationship in one of said paths, a second similar pair of inductorssimilarly connected in the other of said paths and an impedance branchincluding a capacitor and a resistor in parallel connected from a pointin said one patin between said rst pair of inductors to a point in saidother path between said second pair of inductors, an inductor in one ofsaid pairs being connected to an inductor in the other of said pairsthrough an added resistance.

17. A network in accordance with claim 12 and an added capacitor.connected in shunt at one end of the network, the capacitance ofsaidadded capacitor being proportioned with respect to the leakageinductance of said inductors and the stray capacitances to provide alow-pass fllter having a cut-oil.' above the highest frequency to betransmitted by the network.

18. A network in accordance with claim 12 in which inductance is addedin series with one of said inductors in each of said pairs, the value ofsaid lnductance being proportioned with respect to a negative reactivecomponent associated with one of said load impedances to provide a low'-pass lter having a cut-off above the highest frequency to betransmittedl by the network.

SIDNEY DARLINGTON.

Certilcate of Correction Patent No. 2,301,023. l November 3, 1942.SIDNEY DARLINGTON It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows: Page 2, first col-umn, lines 24, 25, and .26, and page 3,first column, lines 5, 6; and 7, for that portion of the equation readmgh I IF F 2 (JE-1) ad (x/-l) and that the said Letters Patent should beread with these corrections therein that the same may conform to therecord of the case in the Patent Offce.l

Signed and sealed this 11th day of January, A. D. 1944.

[SEAL] HENRY VAN ARSDALE,

Acting Uommiss'iouar of Patents.

